Gallium has demonstrated pharmaceutical value for the treatment of many human and animal disorders, including hypercalcemia, cancer, and especially certain widespread degenerative or metabolic bone diseases such as osteoporosis and Paget's disease. For example, numerous clinical studies have shown gallium to have antineoplastic activity, as well as the ability to reduce abnormally high bone turnover in Paget's disease (reviewed in Bernstein, Therapeutic Gallium Compounds, in Metallotherapeutic Drugs and Meta-Based Diagnostic Agents: The Use of Metals in Medicine 259-277 (Gielen and Tiekink eds., 2005)). Gallium is currently approved for use in the United States as a citrate-chelated gallium nitrate solution for intravenous infusion (Ganate®) to treat hypercalcemia of malignancy.
In spite of its established utility, however, the use of gallium in the treatment of such diseases is hampered by the fact that ionic gallium, in the form of salts such as nitrate and chloride, lacks high bioavailability when delivered orally. The low bioavailability of orally delivered gallium salts requires that either impractically large doses of orally delivered gallium be administered to the patient or that the gallium be administered via non-oral means (e.g., intravenous delivery). At present, the oral delivery of such gallium salts is not believed to be practical with chronic conditions such as osteoporosis and Paget's disease due to their low bioavailability.
Efforts have been made to increase the bioavailability of orally administered gallium, particularly through chemical complexing. Several gallium complexes have been identified that demonstrate increased oral bioavailability, including, e.g., gallium maltoate (see, e.g., Bernstein et al., Metal-Based Drugs 7:33-47 (2000); U.S. Pat. Nos. 5,258,376; 5,574,027; 5,883,088; 5,968,922; 5,998,397; 6,004,951; 6,048,851; 6,087,354)) and gallium 8-quinolinolate (see, e.g., Collery et al., Anticancer Res. 16:687-692 (1996); U.S. Pat. No. 5,525,598; European Patent No. EP 0 599 881; International Application No. PCT/EP92/01687). Other therapeutic gallium complexes are described in, e.g., Arion et al., J. Inorg. Biochem. 91:298-305 (2002); Chitambar et al., Clin. Cancer Res. 2:1009-1015 (1996); Stojilkovic et al., Mol. Microbiol. 31:429-442 (1999); U.S. Pat. Nos. 5,196,412; 5,281,578; and International Application No. PCT/US91/03599. There is a continuing need, however, for the development of new pharmaceutical gallium compositions, particularly gallium complexes having enhanced oral bioavailability.